The E90 M3 has provisions for folding T-hooks in the trunk. For those of us who drink high-end bier or occasionally travel with just a few plastic grocery bags, these are nice to use as hangers so your bier doesn't get beaten to death rolling around the trunk in corners.

There are provisions for 2 hooks. The hooks use 2 expanding rivets and mate with a pair of stamped holes in the trunk sheet metal.

Does anyone know what this black plastic clip is for?

Install the T-hook by mating the expanding rivets to the trunk and then pushing the locking pins into place. On my car, the larger pin did not grip as tight as it could. I wrapped the exampling fingers on the larger rivet with a few rounds of electrical tape, then installed it. Very secure, certainly good enough for a few half liters.